This is a tough call. I had an RS1-B system for years until Infinity replaced it with the IRS Beta, which is a much better system. Still, the RS-1B is no slouch. $4k seems like a steep price, though, even if it's in very good shape. Is the x-over/servo included in the price and does it work properly? If not, I'd walk away.
You might also want to consider that if you get the RS-1B, you'll need to upgrade some of your other gear to get the best out of it, especially your preamp. If you need parts in the future - other than refoaming the woofers - you should know they are rather rare and pricey.
You might also want to consider that if you get the RS-1B, you'll need to upgrade some of your other gear to get the best out of it, especially your preamp. If you need parts in the future - other than refoaming the woofers - you should know they are rather rare and pricey.